Convert CONFIG_ROCKCHIP_USB2_PHY to Kconfig

This converts the following to Kconfig:
   CONFIG_ROCKCHIP_USB2_PHY

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/chromebook_minnie_defconfig b/configs/chromebook_minnie_defconfig
index 0565c03..ff94a4d 100644
--- a/configs/chromebook_minnie_defconfig
+++ b/configs/chromebook_minnie_defconfig
@@ -74,6 +74,7 @@
 CONFIG_ROCKCHIP_SPI=y
 CONFIG_SYSRESET=y
 CONFIG_USB=y
+CONFIG_ROCKCHIP_USB2_PHY=y
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_MANUFACTURER="Rockchip"
 CONFIG_USB_GADGET_VENDOR_NUM=0x2207